ohms...what comes first, head or cab?

After 25+ years of owning-playing through combo amps only, I'm ready to "graduate" to the head+cab option. Coming from the combo setup, I've never had to understand matching ohms. So I am at the starting line...which to buy first? Do I buy the head at a "fixed" ohm setting and then attempt to "match" the cab ohms to the head? Or does this make any difference at all? I am presently looking at matching a GK 400RB-IV head with either (1) 2-12 Avatar cab OR (1) 2-10 + (1) 1-15 Avatar cabs. How do I correctly match these for max efficency?

You are going to want to buy two 8ohm cabs for that head, and plug each cab into each speaker jack on the back of the amp. If you bought two 4ohm cabs, and did this, you'd drop your amp down to a 2 ohm load which it might not work good under. You could also buy one 4ohm cab like a 4x10 and use it with the head.

I would say on the norm, most bass amps/heads max out their wattage at 4ohms. If you only want to use 1 cab(I.E. a 4x10 or 8x10), make it a 4ohm cab. If you want to use 2 cabs(I.E. a 4x10/1x15 or a 2x10/1x15), make sure they are 8ohm each. The 2 8ohm cabs will result in a 4ohm load. This is on the norm unless you're using a stereo power amp and then things get a little complicated.

For solid state amps, never go below the minimum rated impedance. It should say what that is right on the amp, usually near the speaker jacks. For instance it'll say "minimum impedance 4 ohms". Then you'll know that you can only use two 8 ohm cabs with it, and adding a third will probably fry your amp. On the other hand, you can safely use a cab with a higher impedance (like 16 ohms), or even no load at all, all that means is you won't have as much power being delivered to the speakers.

For tube amps, it's different, you're safe anywhere between half and double the rated impedance. So if the amp says "8 ohms" output, you can use cabs anywhere between 4 and 16 ohms and be relatively safe (from an electrical standpoint). For tube amps, operating at a lower impedance is better than not having any load at all.

Either way, you want to make sure that the cabs you use will keep your amp happy. If you know you're going to be using four 8-ohm cabs, you're going to need an amp that can handle a 2 ohm load. For a solid state amp, that means the "minimum rated impedance" has to be 2 ohms or less. For a tube amp, it means the output transformer has to have a 2 ohm tap, or in a pinch you can use the 4 ohm tap.